This invention concerns electroluminescent devices and a way of fabricating these, in particular devices which have conjugated polymers as emitting layers.
Electroluminescent devices of the type which this invention concerns, have frequently been reported. As examples of this one can mention PCT/WO 90/13148, PCT/WO 9203490, PCT/WO 92/03491, PCT/WO 93/14177, PCT/WO 94/03030, PCT/WO 94/03031 and U.S. Ser. No. 317,169. These devices have been developed to a considerable extent and new useful fields of application have been found, since the report made by J H Burroughes et al, Nature 347, 539-541 (1991). Thus, different researchers have described improvements which have resulted in that these devices emit light of any color throughout the entire visible spectrum G. Grem et al., Advanced Materials 4, 36-37 (1992); D Braun et al., Applied Physics letters 58, 1982-1984 (1991); and C Zhang et al., Journal of Electronic Materials 22, 413-17 (1993). N. C. Greenham et al. have reported on high quantum efficiency devices in Nature 365, 628-630 (1993), while Goran Gustafsson et al. have reported on the possibility of fabricating large area and flexible light emitting devices.
Conjugated polymers, being organic macromolecules, have offered new possibilities, which earlier have not been obtainable. One advantage of using conjugated polymers as materials in light emitting devices is that the layers can be processed and constructed using solvents, see the PCT publications mentioned above and A. Assadi et al., Applied Physics Letters 53, 1995 (1998) and Ji H. Burroughes et al., Nature 335, 137 (1988). Another advantage is that small chemical modifications of the polymers results in a tunability of the band gap (the color of the emitted light) B. H. Xui, Macromolecules 26, 4457 (1993) and R. E. Gill et. al. Advanced Materials 6, 132 (1994).
If one would like to fabricate devices, each of which consists of more than one emitting layer or which consists of one or more barrier layers, one has, to prevent the already existing layers from being damaged during the fabrication of the next layer placed on top thereof, needed to add different layers from different solvents which not destroy the underlying layer/layers. This problem is particularly obvious, if one would like to fabricate devices each of which consists of layers with thicknesses in the range of 20-100 nm. This restriction of the fabrication procedure is sometimes unfortunate, since it limits the freedom of choice when combining different types of conjugated polymers for the different layers.